This article is from the Stretching FAQ, by Brad Appleton Brad_Appleton@ivhs.mot.com with numerous contributions by others.
This stretch is very good for working toward a side (chinese) split (See
"4.13.3 - The Side Split"). This exercise should be performed *after* you
have stretched each of these areas individually with prior stretches (like
the ones mentioned above).
Start by lying down with your back flat on the ground and your legs
straight together in the air at a 90 degree angle. Try to have your legs
turned out so that your knees are facing the side walls more than they are
facing your head. Slowly bring your legs down to the sides, keeping your
legs straight and turned out. When you reach the point where you cannot
bring them down any further into this "lying" side split position, leave
them there.
Now for the stretch: With your feet both flexed or both pointed (your
choice) use your arms to reach in and grab your legs. Each arm should grab
the leg on the same side. Try to get a hold of the leg between the ankle
and the knee (right at the beginning portion of the calf that is closest to
the ankle is almost perfect). Now, exhale and use your arms to gently but
steadily force your legs down further and wider (keeping the legs straight)
getting closer to the lying side-split position (where, ideally, your
kneecaps would be "kissing" the floor). Hold this position and keep applying
steady pressure with your arms for about 20 seconds.
For the isometric stretch, you do the same thing as the passive stretch
except that, as you use your arms to force your legs wider, use your inner
and outer thigh muscles to try and force your legs back up together and
straight (like a scissors closing), but apply enough resistance with your
arms so that no motion takes place (this can be tough since your legs are
usually stronger than your arms). You may find that you get a much better
stretch if you use a partner (rather than your own arms) to apply the
necessary resistance.
 
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